The present invention relates to a signal converting device and a digital transmitting apparatus, and more particularly to a digital transmitting apparatus having large dynamic power range.
In wireless communication system, such as the third generation (3G) mobile communication system, a high dynamic range of power is required. For example, a cellular may be required to output power with dynamic range of 78 dB. Normally, an on-chip transmitter capable of generating power range from 0 dBm to −78 dBm may comprise various components, such as digital-to-analog converter (DAC), low pass filter, programmable gain amplifier, modulator, and pre-driver, and those components are complicated and may occupy a large area of the transmitter. Therefore, replacing the transmitter by an RF power DAC, such as a digital transmitter using switch-mode power amplifier, may relax the hardware complexity. However, the dynamic range of the digital transmitter is limited by the minimum size of the semiconductor process. For example, the lowest power generated by one switch-mode power amplifier with the minimum size may only be −45 dBm, which is far higher than the required −78 dBm. Therefore, how to generate a small level of power by a switch-mode power amplifier to extend the dynamic range of a digital transmitter is an urgent problem in the wireless communication system.